LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Subaru >> 2017 >> Outback 2.5i Limited >> Repair and Diagnosis (Single Page) >> Engine Performance >> System >> Engine Control System (Diagnostics) (P000A To P015B) (H4DO) >> Diagnostic Procedure with Diagnostic Trouble Code (DTC) >> DTC P0031: A/F/O2 Heater Control Circuit Low Bank 1 Sensor 1 >> Notes
April 5, 2026: LEMON Manuals is launched! Read the announcement.

DTC P0031: A/F/O2 Heater Control Circuit Low Bank 1 Sensor 1: Notes

DTC detecting condition: 

Immediately at fault recognition

CAUTION:
  • After servicing or replacing faulty parts, perform Clear Memory Mode Ref. to OPERATION , Clear Memory Mode, and Inspection Mode Ref. to PROCEDURE , Inspection Mode.
  • Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram: 

Engine Electrical System Ref. to WIRING DIAGRAM , Engine Electrical System.

Fig 1: A/F And O2 Heater Control Circuit Low Bank 1 Sensor 1 - Wiring Diagram
G11583295Courtesy of SUBARU OF AMERICA, INC.
Step Check Yes No
1 CHECK POWER SUPPLY TO FRONT OXYGEN (A/F) SENSOR. 
  1. Turn the ignition switch to OFF.
  2. Disconnect the connectors from front oxygen (A/F) sensor.
  3. Turn the ignition switch to ON.
  4. Measure the voltage between front oxygen (A/F) sensor connector and engine ground.

Connector & terminal 
(E24) No. 2 (+) - Engine ground (-): 		 Is the voltage 10 V or more? Go to step 2  . Repair the power supply line.
NOTE: In this case, repair the following item:
  • Open circuit in harness between A/F, oxygen sensor relay connector and front oxygen (A/F) sensor connector
  • Poor contact of A/F, oxygen sensor relay connector
  • Poor contact of coupling connector
  • Malfunction of A/F, oxygen sensor relay
2 CHECK HARNESS BETWEEN ECM AND FRONT OXYGEN (A/F) SENSOR CONNECTOR. 
  1. Turn the ignition switch to OFF.
  2. Disconnect the connector from ECM.
  3. Measure the resistance between ECM connector and front oxygen (A/F) sensor connector.

Connector & terminal 
(B158) No. 5 - (E24) No. 1: 		 Is the resistance less than 1 Ω? Go to step 3  . Repair the open circuit of harness between ECM connector and front oxygen (A/F) sensor connector.
3 CHECK GROUND CIRCUIT FOR ECM. 
Measure the resistance of harness between ECM connector and chassis ground.
Connector & terminal 
(E158) No. 61 - Chassis ground: 
(E158) No. 62 - Chassis ground: 
(E158) No. 63 - Chassis ground: 
(E158) No. 64 - Chassis ground: 
(E158) No. 65 - Chassis ground: 		 Is the resistance less than 5 Ω? Go to step 4  . Repair the open circuit in harness between ECM connector and engine ground.
4 CHECK FRONT OXYGEN (A/F) SENSOR. 
Measure the resistance between front oxygen (A/F) sensor terminals.
Terminals 
No. 1 - No. 2: 		 Is the resistance 2 - 3 Ω? Repair the poor contact of ECM connector. Replace the front oxygen (A/F) sensor. Ref. to FRONT OXYGEN (A/F) SENSOR
scientia non olet · About LEMON Manuals